Run by the US Department of Defense, responsible for making unclassified orbital elements
available to the public. This organisation has taken over the task from the OIG which was run by NASA.
You will need to get an account to access the data.

A good page containing elements of satellites organised by category, and stored in ZIPped files for easy
downloading. Also contains elements for classified satellites, and a single large file with the
most recent elements of most satellites.